The present invention relates to a method for measruing the wall thickness of a workpiece which is in the form of a half cylinder and which is formed by dividing a cylinder along its longitudinal axis into two parts. The invention further relates to an apparatus for practicing this method.
Usually, a bearing bushing is manufactured according to a method in which two rectangular plates are bent into two bearing halves each in the form of a half cylinder and the two bearing halves thus obtained are butt-welded. In such manufacture, it is necessary to reject bearing halves which have an unacceptable wall thickness. Accordingly, various methods for measuring the wall thickness of a half-cylinder-shaped workpiece and apparatuses for practicing such methods have been proposed in the art.
A workpiece wall thickness measuring apparatus of this type is disclosed in Japanese Laid-Open Patent Application No. 98352/1975 filed by the present applicant. In this apparatus, a half-cylinder-shaped workpiece is positioned between upper and lower measruing units in such a manner that the inner surface of the workpiece faces downwardly. The upper and lower measuring units are then brought into contact with the workpiece to measure the wall thickness. In this measurement, the lower ends of the legs of the workpiece rest on the measuring table. Therefore, if workpieces set on the measuring table are variable in height, then the operating strokes required for the upper and lower measuring units to contact the various workpieces are also variable. The variations in height of the workpieces may be due to the fact that the rectangular plates from which the workpieces are manufactured are variable in dimensions or the rectangular plates are unsatisfactorily bent.
Measurement of the wall thickness of a workpiece with the upper and lower measuring units is, in general, carried out by detecting the amounts of movement of the measuring units with differential transformers. The differential transformers operate to convert a mechanical displacement into an electrical output signal. Unfortunately, the linear range of the input-output conversion characteristics is generally small for available differential transformers. Therefore, because variations in the measuring units result in variations in operating stroke, sometimes it is impossible using differential transformers to accurately detect the amounts of movements of the measuring units, and accordingly the wall thickness measurement accuracy is sometimes unacceptably low.